NO. 2345 



CONTRIBUTIONS TO MINERALOGY— SHANNON. 



449 



brown to almost black, in thin section and apparently tourmaline. 

 This tourmaline is unusual in that it shows in places quite well- 

 defined cleavage parallel to the vertical axis. The rock is somewhat 

 cavernous and rusted as from the oxidation of pyrite. Under the 

 microscope the brown mineral is colorless except where stained by 

 infiltrated limonite. The indices of refraction as determined by im- 

 mersion are: 



a=1.644±.003. 

 7=1.660±.003. 



The elongation is positive, and in all the specimens collected the 

 extinction is exactly parallel and not inclined. The mineral thus 

 appears to unquestionably be anthophyllite rather than cumming- 

 tonite. A cleavage fragment was mounted on a one-circle goniom- 

 eter and the cleavage surfaces were sufficiently bright to give dim, 

 broad reflections of the signal, permitting measurements accurate 

 perhaps to 2 degrees. A prismatic and a (b) pinacoidal cleavage 

 were represented and gave the following angles : 



Anthophyl- 

 lite, Dana. 



771 /\ m'' 

 m /\ m^ " 

 m /\b 

 m /\b 



125 37 



54 23 



62 49 



62 49 



Crystallographically and optically the numeral thus agrees with 

 anthophyllite, although the mode of occurrence is the same as that 

 of the cummingtonite of the type locality in the adjoining township 

 of Cummington, and the Chesterfield bed seems to represent a con- 

 tinuation of the cummingtonite bed. Just what relationships exist 

 between the orthorhombic and the monoclinic amphiboles in these 

 beds is a subject worthy of some study. It seems not improbable 

 that the monoclinic form is the original less stable mineral and that 

 it is paramorphosed to anthophyllite under the influence of condi- 

 tions near the surface. Upon analysis the mineral yielded the fol- 

 lowing results, the ferric oxide being probably extraneous and de- 

 rived by infiltration from decomposing pyrite. The mineral is ref- 

 erable to the gedrite variety of anthophyllite. 

 181404— 21— Proc.N.M.vol.58 29 



